1. Field of the Invention
The present invention relates to an image forming apparatus and, particularly, to an image forming apparatus for forming an image on a continuous paper.
2. Description of the Related Art
There is a conventionally known tandem-type image forming apparatus for forming toner images of plural colors of yellow, magenta, cyan, and black on plural image carrying members arranged along a continuous paper transporting path and for transferring the formed toner images of the plural colors on the image carrying members onto a continuous paper being transported to the apparatus such that they are overlaid, thereby forming an image on a continuous paper.
In such an image forming apparatus, a technique is known in which before an image based on each set of image data included in a print job is formed, pattern images of the different colors corresponding to each of the sets of data image data of the print job are formed on a continuous paper and, on the basis of a correction amount computed based on the result of detection of a shift amount of the formed pattern images of each of the colors, an image formation position for the images of respective colors according to the image data is corrected. In such a manner, images of the plural colors based on the image data can be formed so as to be overlaid on a continuous paper without a color shift by correcting the positional deviations of the images of the colors.
The image forming apparatus has a problem such that when a continuous paper is also transported continuously between print jobs, the continuous paper on which no image is formed is transported before an image based on image data for the next print job is formed, such that a blank area is excessively produced on the continuous paper. To solve such a problem, there is a known technique of back-transporting a print start page of a continuous paper to a position where an image can be formed, and thereafter, forming an image (for example, refer to Patent Publication No. 3,317,908). By applying the technique and back-transporting a continuous paper to an image formation start position for the next print job when a print job is finished, excessive formation of a blank area can be suppressed.
Although excessive formation of a blank area on a continuous paper can be suppressed by back-transporting the continuous paper when a print job is finished by applying the related art, a method of correcting and forming a pattern in a case where a pattern image in a back-transported area is detected has not been considered. In the case of detecting, after the continuous paper is back-transported, a shift amount of a pattern image that is formed before the continuous paper is back-transported and that is already formed on the continuous paper and then correcting the image formation position of each color on the basis of the detected shift amount, a problem occurs of an erroneous correction being made. The erroneous correction occurs when a pattern image of before the continuous paper is back-transported is used in the case where the pattern image forming conditions at the time of forming a pattern image before the continuous paper is back-transported are different from those at the time of forming a pattern image after the continuous paper is back-transported, due to a transport shift of the continuous paper, elongation of the continuous paper, standby time, and the like. A problem also occurs such that when a continuous paper is back-transported without considering the pattern image position before the continuous sheet is back-transported, and then forming a pattern image, pattern images overlap each other and an erroneous correction is made.